1. Technical Field of the Invention
The present invention relates to a method of determining the volumetric level of barrier fluid in a mechanical seal pressure vessel.
2. Description of the Prior Art
In an American Petroleum Institute (“API”) Plan 53B mechanical seal support system, a bladder is pre-charged with nitrogen to a predetermined pressure. Once it has been pre-charged with gas, it is filled with barrier fluid to the required pressure: 2 barg greater than maximum product pressure. During operation, the accumulator is isolated from the gas and the barrier fluid source.
In normal operation, it is anticipated that there will be leakage from the mechanical seal, which will reduce the barrier fluid volume in the accumulator over time. As the fluid level in the accumulator drops, pressure will be lost. This can result in the contamination of the barrier fluid by the product and may lead to premature failure of the seal. In the worst case scenario, the product could escape and a fire could result from the over-heating of the seal. If the leakage rate of the seal is known, it is possible to estimate when the barrier fluid will require refilling. It is a common industry standard that it can be expected for the barrier fluid to require refilling every 28 days or more, although this may vary depending on the volume of barrier fluid, the seal leakage rate and the alarm strategy employed. Also, due to unforeseen variations in seal performance or environmental conditions, it is sometimes not possible to predict with accuracy when refilling will be required and so controls need to be implemented.
The existing controls come either in the form of pressure transmitters connected to the accumulator, which highlight when the pressure of the barrier fluid has reduced to a particular level and the accumulator needs refilling, or a combination of pressure and temperature transmitters connected to the site distribution control system (“DCS.”) The combination of a pressure and temperature transmitter is sometimes preferable due to changes in ambient temperature affecting the accuracy of the pressure readings. An algorithm in the site DCS system utilizes the readings from both the transmitters to calculate the volume of barrier fluid left in the accumulator. If a DCS is not available, it has been proposed that individual single loop controllers are used on each 53B system. Any control involving an electrical component will require ATEX approval with the result that these two solutions can be very expensive and difficult for the site to implement. It is therefore the purpose of this invention to provide an alternative means for fluid level detection.